Solid core golf balls are well known in the art. Typically, the core is made from polybutadiene rubber material, which provides the primary source of resiliency for the golf ball. U.S. Pat. Nos. 3,241,834 and 3,313,545, which are incorporated herein by reference in their entirety, disclose the early work in polybutadiene chemistry. It is also known in the art that increasing the cross-link density of polybutadiene can increase the resiliency of the core. The core is typically protected by a cover from repeated impacts from the golf clubs. The golf ball may comprise additional layers, which can be an outer core or an inner cover layer. One or more of these additional layers may be a wound layer of stretched elastic windings to increase the resiliency of the ball.
A known drawback of polybutadiene cores cross-linked with peroxide and/or zinc diacrylate is that moisture adversely affects this material. Water moisture vapor reduces the resiliency of the core and degrades its properties. A polybutadiene core will absorb water and loose its resilience. Thus, preferably a golf ball core is covered quickly to maintain optimum ball properties. The cover is typically made from ionomer resins, balata, and urethane, among other materials. The ionomer covers, particularly the harder ionomers, offer some protection against the penetration of water vapor. However, it is more difficult to control or impart spin to balls with hard covers. Conventional urethane covers, on the other hand, while providing better ball control, offer less resistance to water vapor than ionomer covers.
Prolonged exposure to high humidity and elevated temperature may be sufficient to allow water vapor to invade the cores of some commercially available golf balls. For example at 110° F. and 90% humidity for a sixty day period, significant amounts of moisture enter the cores and reduce the initial velocity of the balls by 1.8 ft/s to 4.0 ft/s or greater. The change in compression may vary from 5 to about 10 or greater. The absorbed water vapor also reduces the coefficient of restitution (COR) of the ball.
Several prior patents have addressed the water vapor absorption issue. Commonly owned U.S. Pat. No. 6,632,147 B2, which is incorporated herein by reference in its entirety, describes a barrier layer in the form of an intermediate layer that has a moisture vapor transmission rate lower than that of the cover. The moisture vapor barrier layer may comprise nanoparticles, flaked glass, leafing or non-leafing metal flakes (e.g., aluminum flakes, iron oxide flakes, copper flakes, bronze flakes) or ceramic particles to increase the layer's resistance to the transmission of moisture through the layer. The primary ingredient of the barrier layer is made from a material or composition, such as polybutadiene, natural rubber, butyl-based rubber, acrylics, trans-polyisoprene, neoprene, chlorinated polyethylene, and balata. Furthermore, in one example, the intermediate layer is made from a multi-layer thermoplastic film having a base layer and a coating layer. The base layer includes polyethylene teraphthalate, polybutylene teraphthalate, polyethylene naphthalate, polycyclohexanedimethylene teraphthalate, and polyethylene naphthalate bibenzoate and the coating layer includes polyvinylidene chloride, ethylene vinyl alcohol, modified polyester, silicon oxide, and one or more copolyesters prepared from dicarboxylic acids and diols or its derivatives. The vapor barrier layer can also have high specific gravity to form a ball with high moment of inertia.
U.S. Pat. No. 5,820,488 discloses a golf ball with a solid inner core, an outer core, and a water vapor barrier layer disposed there between. The water vapor barrier layer can be a polyvinylidene chloride (PVDC) layer or a vermiculite layer. Commonly owned U.S. Pat. Nos. 5,885,172 and 6,132,324 disclose, among other things, a golf ball with a polybutadiene or wound core with an ionomer resin inner cover and a relatively soft urethane outer cover. The hard ionomer inner cover offers some resistance to water vapor penetration and the soft outer cover provides the desirable ball control. It is also desirable to minimize the water barrier layer such that other properties of the ball are unaffected. These references are incorporated herein by reference in their entireties.
Known methods for forming the moisture vapor barrier layers include using pre-formed semi-cured shells. A quantity of mixed stock of the preferred moisture vapor barrier material is placed into a compression mold and molded under sufficient pressure, temperature, and time to produce semi-cured, semi-rigid half-shells. The half-shells are then placed around a core (solid or wound) and the sub-assembly is cured in another compression molding machine to complete the curing process. A cover is then formed on the sub-assembly by any known method to complete the fabrication of the ball. Another known method is the sheet stock and vacuum method. Thin sheets of the mixed stock of the preferred moisture vapor barrier material are placed on the upper and lower platens of a compression-molded press. Suction force from a vacuum keeps the sheets tight against the mold cavities. A core (solid or wound) is placed in the bottom cavity above the vacuumed sheet. The sub-assembly is then cured in a compression molding press to cure the sub-assembly. A cover is then formed on the sub-assembly by any known method to complete the fabrication of the ball. Another known method is the rubber injection molding, wherein mixed stock of the preferred moisture vapor barrier material is fed into an injection molding barrel and screw. The stock is then injected through a nozzle into a mold cavity and surrounds a core (solid or wound). The sub-assembly is then heated under pressure to cure the sub-assembly. A cover is then formed on the sub-assembly by any known method to complete the fabrication of the ball. Other manufacturing techniques include spraying, dipping, vacuum deposition, reaction injection molding, among others. All of above-described methods which incorporate a water vapor barrier layer on the surface of the core have shortcomings, such as being expensive, not fully cover the pores of the core, and time consuming.
Thus, there remains a need for golf improved methods for applying the water vapor barrier layer on to the core of the golf ball.